1. Field of the Invention
The present invention relates to selected compositions useful as a lift-off photoresist in a bilayer metal lift-off process. In particular, this invention relates to specific compositions useful for that purpose that include at least one solvent, at least one polyglutarimide resin and at least one selected actinic-absorbing dye useful as dissolution rate modifier.
2. Brief Description of Art
The additive process of depositing patterned metal films onto microelectronic substrates is known as the lift-off process or metal lift-off process. There are several variations of this lift-off process. The most widely used lift-off processes involve a bilayer lithographic process (sometimes also referred to as a "bilevel" process). Such bilayer lift-off processes have been used to deposit the metallic "read-stripe" in the manufacture of thin film heads for magnetic hard drives and in the fabrication of the gate oxide for GaAs Field Effect Transistor (FET) devices. Variants of these bilayer lift-off processes are described in detail in European Patent Application No. 0341843 (assigned to International Business Machines Corp.) and U.S. Pat. No. 4,814,258 (assigned to Motorola Inc.).
In bilayer lift-off processes, a solution of a non-imaging lift-off resist (LOR) is first deposited by spin-coating to form a uniform thin film on top of a substrate to be metallized. The LOR layer is then soft-baked by heating at a sufficiently high temperature to remove most of the solvent contained in it. A conventional positive-imaging resist layer is then deposited on top of the LOR. The top resist and the lower LOR layer should not be intermixed. Therefore, the LOR should have a low solubility in conventional positive resist solvents.
After a second soft-bake to remove most of the residual solvent in the top resist layer, a pattern is transferred from a mask to the top resist film using a conventional microlithographic imaging tool such as a contact-proximity printer or stepper. The exposed areas in the top resist layer represent the areas to be metallized. The exposed resist is developed with an aqueous developer through to the LOR layer, which then dissolves both vertically through to the substrate and laterally to penetrate a small predefined distance into the adjacent unexposed areas of the photoresist layer. This lateral dissolution produces a controlled degree of undercut in a development time which is neither too long to make the process impractical or to remove too much unexposed photoresist, or too short to make the process irreproducible. In one variation of the process, often referred to as the PCM (portable conformable mask) variation, the underlying LOR is photosensitive in the deep ultra-violet (DUV) spectral range and the positive-imaging top resist is of the novolak-diazo-naphthoquinone type. The latter absorbs in the DUV and acts as a mask to an intermediate DUV flood exposure. This renders the lower LOR layer more soluble in a selected developer in the exposed areas that are to be removed during the development process. It is preferred to avoid a DUV intermediate exposure step and rely instead on the LOR having the desired rate of dissolution in the positive imaging resist developer. Moreover, this PCM process cannot be used with a positive top resist of the chemically amplified type designed to be photosensitive to DUV wavelengths.
After the desired degree of undercut is developed in the LOR layer, the metal layer is blanket-deposited by sputtering. The undercut ensures a discontinuity between the metal on top of the resist and the metal in the trench formed by the lithographic process. By this means, upon subsequent stripping of the remaining top photoresist and the LOR, the metal deposited on top of the resist is cleanly separated from the metal deposited on the substrate, ensuring consistent profiles and critical dimensions of the metal pattern. The degree of undercut, and hence the lateral dissolution rate, must be carefully controlled.
Partially or fully imidized acrylic polymers referred to as polyglutarimides, especially polydimethylglutarimide (PMGI), have been described in U.S. Pat. No. 4,524,121 (assigned to Rohm and Haas Co.). Polyglutarimide refers to a class of polymers containing partially cyclized imide and N-alkyl imide moeties and uncyclized polymethacrylate, in which the degree of cyclization as well as the ratio of N-alkyl to N-H can vary widely depending on the starting materials and the process used in the preparation. In the case where the alkyl group is methyl, the polymer is more correctly referred to as polydimethylglutarimide, or PMGI. If PMGI is made from polymethacrylic acid or a PMMA/methacrylic acid copolymer, (uncyclized) poly(methacrylicacid) units may also be present. PMGI polymers for lift-off applications are generally found to comprise about 65%-80% or more of cyclized imide moieties of which about 50-60% are N-H and the remainder N-methyl substituted. These compounds have several desirable properties, especially good solubility in aqueous bases typically used for the development of conventional positive resists, and poor solubility in positive resist solvents such as ethyl lactate, 2-heptanone and propylene glycol methyl ether acetate, which make them suitable for use in lift-off resists for bilayer lift-off process applications. Additionally, their solubility may be increased by exposure to high energy radiation such as deep ultra-violet (DUV) or electron beam.
The basic reaction to form poly(N-alkylimides) from the reaction of poly(methylmethacrylate)(PMMA) or poly(methacrylic acid) with an amine is disclosed in Graves U.S. Pat. No. 2,146,209, (assigned to E. I. du Pont de Nemours & Co.), see German Patent No. 1,077,872 and Makromol. Chem. 96, 227 (1966).
European Patent Application No. A0275918 (assigned to Verdril S.p.A.) discloses a solution process for making imidized acrylic polymers by reaction of acrylic resin with an amide
U.S. Pat. No. 4,689,243 (assigned to Mitsubishi Rayon Co.) discloses a process for forming polyglutarimide polymers by reaction of a solution of PMMA with ammonia or an amine, followed by separation of the polymer from non-polymeric reaction products and solvents under vacuum in a vent extruder. As described in U.S. Pat. No. 3,284,425, the same reaction is carried out in a suspending solvent in an autoclave.
In any practical lift-off process, it is desirable to adjust and maintain precise control of the dissolution rate of the lift-off resist layer, so that the required degree of undercut is always obtained in a relatively short time using a developer which is compatible with, and provides a wide process latitude for the imaging positive photoresist layer.
Commercially available PMGI has been manufactured by the process described in U.S. Pat. No. 4,246,374 (assigned to Rohm and Haas). In this process, poly(methyl methacrylate) (PMMA) is imidized with ammonia gas in an extruder at high pressure and relatively high temperature. This reaction is practical only if the weight-average molecular weight (M.sub.w) of the starting PMMA is sufficiently high (i.e. greater than 60,000 and typically 60,000 to 120,000). The resulting polymer should also contain about 20-35% of unreacted methacrylate moieties and about 30-60% of the nitrogen atoms on the imide groups should be methylated. The percentage of the remaining imide groups (N-H) determines the alkaline solubility. PMGI resins produced by this process have a fairly narrow range of alkaline solubility. This limitation creates the need for other methods of modifying the dissolution rate of these PMGI resins.
One such method is to reduce the molecular weight of PMGI by exposing the polymer to DUV radiation. This method has been described in U.S. Pat. No. 4,636,532 (assigned to Shipley Co.) By this means, the dissolution rate of PMGI, and hence the rate of undercut, can be increased to some extent. However, the amount of increase in the dissolution rate may be insufficient for certain lift-off processes requiring a relatively large rate of undercut to be useful with certain developers.
Additionally, the dissolution rate and hence the degree of undercut can also be adjusted somewhat by changing the conditions under which the spin-coated LOR film is soft-baked, especially the bake temperature. This arises because the dissolution rate of solvent-cast PMGI, like other polymers, is strongly dependent upon the concentration of solvent retained in the cast film. However, controlling the dissolution rate by this means is somewhat limited in practice, since other process requirements generally restrict the bake temperature range. For example, in the lift-off process commonly used in the manufacture of thin film heads, the maximum soft-bake temperature is generally about 160-170.degree. C. in order to minimize adverse effects on the magnetic properties. Moreover, the rate of decrease of the dissolution rate of a PMGI LOR with temperature tends to become small above about 190.degree. C., when most of the casting solvent has been removed. For all lift off processes, the recommended minimum bake temperature to produce good reproducibility in a PMGI LOR is about 150.degree. C. Below 150.degree. C., the dissolution rate tends to change very rapidly as a function of the bake temperature, exposure energy, time of development and other process parameters, which results in a narrow process window.
A further method of controlling the dissolution rate has been to change the conditions of the development process, such as varying the type or normality of the developer or the development time. When an advanced-type commercial positive resist is used for the top layer, it is desirable to use a developer which is optimally selected to provide the widest process window when used in conjunction with that photoresist. Such a developer may be of a type or normality which is less suited to achieve the desired rate of undercut in the LOR layer. This may lead to a development time that is too long or too short, or a soft-bake temperature that is too high or too low for optimum undercut. Thus, there is a need for other methods of controlling dissolution rates besides varying the type and normality of developer.
As the size of the features in the metal patterning process decreases, which is the trend in FET device and thin film head technologies, the degree of undercut required for the same development time also decreases, and hence there is a need to reduce rather than increase the PMGI dissolution rate after baking.
It has been determined that the use of lower dissolution rate, high molecular weight PMGI as the resin in an LOR lift-off processes can result in the formation of residue frequently referred to as scum, which retards the lateral dissolution and may give rise to defects in the final device. The propensity for the formation of scum is greater at lower rates of dissolution, especially if the undercut rate is about 0.3 microns per minute or slower, which may be the case if the width of the metal feature of the lift-off process is less than about one micron. The propensity for the formation of scum diminishes if a low molecular-weight PMGI resin is used; however, such a resin has a relatively high dissolution rate compared with that ideally required for sub-micron processes requiring low rates of undercut.
Additionally, the positive photoresist selected for imaging a fine pattern is usually of an advanced type typically used in the fabrication of sub-micron semiconductor devices. These positive photoresists have been optimized for use with a specific normality of tetramethyl ammonium hydroxide (TMAH) developer (e.g. 2.38% by weight of TMAH in deionized water with or without an added surfactant). However PMGI resins, especially those having a low molecular weight, and, therefore, having a low propensity to scum, even after soft-baking at 200.degree. C., tend to dissolve too rapidly in 2.38% TMAH developer so that it is difficult to obtain the desirable low undercut rate.
There is, therefore, a need to modify the dissolution rate of a lift-off resist, in a manner which results in precise control of the undercut rate, does not produce undesirable scum, permits the choice of a developer composition which is most compatible with the imaging photoresist, and maintains a wide process window for the lift-off process, especially when the degree of undercut required is to be relatively small. The present invention is a solution to this need.
Furthermore, the need for smaller geometries has recently led to a wide interest in the use of reduction steppers as the exposure tools, which combine high resolution imaging, a high throughput, and mask features which are 4 or 5 times larger in dimensions than those to be reproduced, therefore, making it easier to produce the masks. High resolution steppers normally use as the exposing radiation, selected narrow-band wavelengths of light filtered from the output of a mercury arc or mercury-xenon arc lamp. The shorter are the selected wavelengths, the higher is the resolution. Of particular interest for fine geometries such as 0.7 microns and smaller, is the use of an exposure wavelength centered at the mercury i-line at 365 nm. Of further interest for geometries below 0.3-0.35 um is an exposure source using an eximer laser at a wavelength of 248 nm.
It is widely known that a light-wave propagating through one or more thin film layers of transparent or semitransparent materials to a reflecting substrate can interfere with the reflected wave to produce standing waves. The amplitude of these standing waves depends on the reflectivity, the wavelength and the thickness of the films. Standing waves result in a changing exposure of the resist film, which depends critically on the thickness of the resist layer, the LOR layer, and the underlying substrate topography, all of which vary in practice, thus reducing the process latitude. The amplitude of standing waves is high when the wavelength is relatively short and the substrate reflectivity is relatively high, as is the case for most metals, silicon and GaAs. It is, therefore, desirable to increase the absorption of the LOR, such that a thin film will reduce the intensity of the reflected light and hence suppress the standing waves.
The reduction of the amplitude of standing waves by the use of dyes as additives in a conventional imaging photoresist, or in an antireflective coating as an intermediate layer between a photoresist and a reflective substrate, is well-known in the field of micro-lithography. Dyes which are suitable for use in antireflective coatings or as additives to photoresists, especially positive resists may not, however be suitable for use in a bilevel lift-off application because of the different requirements. In particular, it is especially difficult to select a dye which can be added to an LOR at a concentration high enough to provide sufficient absorbance in a thin film of the LOR at a specific actinic wavelength, yet capable of dissolving in a developer which is compatible with the imaging resist, at a rate commensurate with that required for the controlled degree of undercut, and will not form insoluble residues in the undercut regions. The dye must also be non-subliming or non-volatalizing, at temperatures significantly higher than those used to soft-bake positive resists, miscible with a polyglutarimide type polymer, and non-diffusing or non-leaching into a positive photoresist used as the top imaging layer in the lift-off process.
The addition of an actinic wavelength absorbing dye to a PMGI resin is disclosed in U.S. Pat. No. 5,604,073 assigned to International Business Machines Corp. The dye described in that IBM patent is a mono-azo dye, and is used as an adhesion promoter. It has a specific structure which acts as chelating ligand to the surface of a metal with which it may bind. The undercut rates of the disclosed compositions containing the dye are controlled by changing the development time or bake temperature.